This application claims the priority of Korean Patent Application No. 2003-780 filed on Jan. 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to an edge light backlight unit using a light guide panel (LGP) and a point light source.
2. Description of the Related Art
Typically, flat displays are classified into light emission types and light receiving types. An example of a light receiving type flat display is a liquid crystal display. Since the liquid crystal display does not form an image by emitting light by itself, but forms an image by receiving light from the outside, the image displayed on the liquid crystal display cannot be viewed in a dark place. Thus, a backlight unit for emitting light is installed on a rear surface of the liquid crystal display.
According to the arrangement of a light source, the backlight unit can be classified into a direct light type in which a plurality of lamps installed directly under a liquid crystal display directly emit light to a liquid crystal panel, and an edge light type in which a lamp installed at an edge of a light guide panel emits light and the light is transferred to a liquid crystal panel.
The edge light type may use a linear light source or a point light source. A typical linear light source is a cold cathode fluorescent lamp (CCFL) in which electrodes at both end portions are installed in a tube. As a point light source, there is a light emitting diode (LED). The CCFL can emit a strong white light, exhibits a high brightness and a high homogeneity, and makes a large area design possible. However, it is disadvantageous in that the CCFL is operated by a high frequency AC signal and an operational temperature range is narrow. Compared to the CCFL in brightness and homogeneity, the LED does not perform as well. However, the LED has the advantages of being operated by a DC signal, having a long life span and a wide operational temperature range, and capable of being made thin.
A light guide panel used for an edge light backlight unit converts light input through the edge from a linear light source or a spot light source to a surface light and emits the surface light in a vertical direction. A dispersion pattern or holographic pattern is formed on the light guide panel in a print method or mechanical processing method to convert the incident light to a surface light.
FIG. 1 is a perspective view illustrating a conventional edge light backlight unit using a point light source. FIG. 2 is a sectional view of the edge light backlight unit shown in FIG. 1. Referring to FIG. 1, three LEDs 20 are installed at an edge 11 of the light guide panel 10 as point light sources. A holographic pattern 30 to emit light emitted from the LEDs 20 to a light exhaust surface 12 is formed on the bottom surface of the light guide panel 10.
Each of the LEDs 20 emits light toward the edge 11 of the light guide panel 10. Since the LEDs 20 are point light sources, light is emitted within a range of azimuth angles of ±90° with respect to an optical axis, as shown in FIG. 3. Here, an azimuth angle at which light having an intensity (Imax/2) corresponding half the maximum value (Imax) thereof is referred to as a full width half maximum (FWHM). For an LED, the FWHM is typically about ±45°.
The light emitted from the LEDs 20 is input to the light guide panel 10 through the edge 11 and incident on the holographic pattern 30. The holographic pattern 30 having a diffraction grating converts the incident light to a surface light to be emitted toward the light exhaust surface 12 which is an upper surface of the light guide panel 10. The holographic pattern 30 has a certain directionality so that light can be emitted at the highest efficiency when the light is incident at an angle of about 90° with respect to the holographic pattern 30. Also, when the an incident azimuth angle distribution of light incident on the holographic pattern 30 decreases, uniform brightness can be obtained at the light exhaust surface 12. If the brightness of the light exhaust surface 12 is not uniform, a screen appears smeared. In a narrow range of about 1 cm, a brightness change of about 0.9 is detected as a smear. However, when brightness changes gradually from the central portion to the edge portion of the screen, a smear in brightness is not detected even at a change in brightness of about 0.8. Thus, a degree of uniform brightness of 0.8 or more is needed and, in order to obtain a quality image, a degree of uniform brightness of 0.9 or more is needed.
FIG. 4 is a view showing a distribution of light emission by the conventional backlight unit shown in FIG. 1. The light guide panel 10 is divided into three zones: a near portion 40, a middle portion 50, and a far portion 60, sequentially from the edge 11 where the LEDs 20 are installed. In FIG. 4, the middle portion 50 and the far portion 60 have a large light exhaust distribution compared to the near portion 40.
FIG. 5 is a graph showing brightness at the light exhaust surface 12 by the edge light backlight unit shown in FIG. 1. In the graph, a vertical axis indicates brightness and a horizontal axis indicates a light exhaust angle at the light exhaust surface 12 as an FWHM. Three curves C1, C2, and C3 indicate brightness of the near portion 40, the middle portion 50, and the far portion 60 respectively. Referring to FIG. 5, it can be seen that brightness of the near portion 40 is greater than those of the middle portion 50 and the far portion 60. While the FWHM is 20°/20° at the near portion 40, it is 20°/35° at the middle portion 50 and the far portion 60 which appears wider. In 20°/35°, the angle “20°” and the angle “35°” indicate the FWHMs in directions X and Y, respectively.
The irregular brightness results from a fact that the distribution of an incident azimuth angle of light incident on the holographic pattern 30 at the middle portion 50 and the far portion 60 is greater than that of the near portion 40. That is, light having a variety of incident azimuth angles resulting from multiple reflections as shown in FIG. 2 is incident on the holographic pattern 30 in the middle portion 50 and the far portion 60 located far from the LEDs 20. The irregular brightness becomes severe as the distribution of an incident azimuth angle of light emitted from the LEDs 20 and incident on the light guide panel increases.